An acoustic virtual environment means an audible impression with the aid of which the listener to an electrically reproduced sound can imagine that he is in a certain space. Complicated acoustic virtual environments often aim at imitating a real space, which is called auralization of said space. This concept is described for instance in the article M. Kleiner, B.-I. Dalenbäck P. Svensson: “Auralization—An Overviews”, 1993, J. Audio Eng. Soc., vol. 41, No. 11, pp. 861-875. The auralization can be combined in a natural way with the creation of a visual virtual environment, whereby a user provided with suitable displays and speakers or a headset can examine a desired real or imaginary space, and even “move around” in said space, whereby he gets a different visual and acoustic impression depending on which point in said environment he chooses as his examination point.
The creation of an acoustic virtual environment can be divided into three factors which are the modeling of the sound source, the modeling of the space, and the modeling of the listener. The present invention relates particularly to the modeling of a sound source and the early reflections of the sound.
The VRML97 language (Virtual Reality Modeling Language 97) is often used for modeling and processing a visual and acoustic virtual environment, and this language is treated in the publication ISO/IEC JTC/SC24 IS 14772-1, 1997, Information Technology—Computer Graphics and Image Processing—The Virtual Reality Modeling Language (VRML97), April 1997; and on the corresponding pages at the Internet address http://www.vrml.org/Specifications/VRML97/. Another set of rules being developed while this patent application is being written relates to the Java3D, which is to become the control and processing environment of the VRML, and which is described for instance in the publication SUN Inc. 1997: JAVA 3D API Specification 1.0; and at the Internet address http://www.javasoft.com/-products/java-media/3D/forDevelopers/3Dguide/-. Further the MPEG-4 standard (Motion Picture Expert Group 4) under development has as a goal that a multimedia presentation transmitted via a digital communication link can contain real and virtual objects, which together form a certain audiovisual environment. The MPEG-4 standard is described in the publication ISO/IEC JTC/SC29 WG11 CD 14496. 1997: Information technology—Coding of audiovisual objects. November 1997; and on the corresponding pages at the Internet address http://www.cselt.it/-mpeg/public/mpeg-4_cd.htm.
FIG. 1 shows a known directed sound model which is used in VRML97 and MPEG-4. The sound source is located at the point 101 and around it there is imagined two ellipsoids 102 and 103 within each other, whereby the focus of one ellipsoid is common with the location of the sound source and whereby the main axes of the ellipsoids are parallel. The sizes of the ellipsoids 102 and 104 are represented by the distances maxBack, maxFront, minBack and minFront measured in the direction of the ma axis. The attenuation of the sound as a function of the distance is represented by the curve 104. Inside the inner ellipsoid 102 the sound intensity is constant, and outside the outer ellipsoid 103 the sound intensity is zero. When passing along any straight line through the point 101 away from the point 101 the sound intensity decreases linearly 20 dB between the inner and the outer ellipsoids. In other words, the attenuation A observed at a point 105 located between the ellipsoids can be calculated from the formulaA=−20 dB·(d′/d″)where d′ is the distance from the surface of the inner ellipsoid to the observation point, as measured along the straight line joining the points 101 and 105, and d″ is the distance between the inner and outer ellipsoids, as measured along the same straight line.
In Java3D directed sound is modeled with the ConeSound concept which is illustrated in FIG. 2. The figure presents a section of a certain double cone structure along a plane which contains the common longitudinal axis of the cones. The sound source is located at the common vertex 203 of the cones 201 and 202. Both in the regions of the front cone 201 and of the back cone 202 the sound is uniformly attenuated. Linear interpolation is applied in the region between the cones. In order to calculate the attenuation detected at the observation point 204 you must know the sound intensity without attenuation, the width of the front and back cones, and the angle between the longitudinal axis of the front cone and the straight line joining the points 203 and 204.
A known method for modeling the acoustics of a space comprising surfaces is the image source method, in which the original sound source is given a set of imaginary image sources which are mirror images of the sound source in relation to the reflection surfaces to be examined: one image source is placed behind each reflection surface to be examined, whereby the distance measured directly from this image source to the examination point is the same as the distance from the original sound source via the reflection to the examination point. Further, the sound from the image source arrives at the examination point from the same direction as the real reflected sound. The audible impression is obtained by adding the sounds generated by the image sources.
The prior art methods are very heavy regarding the calculation. If we assume that the virtual environment is transmitted to the user for instance as a broadcast or via a data network, then the receiver of the user should continuously add the sound generated by even thousands of image sources. Moreover, the bases of the calculation always changes when the user decides to change the location of the examination point. Further the known solutions completely ignore the fact that in addition to the direction angle the directivity of the sound strongly depends on its wave-length, in other words, sounds with a different pitch are directed differently.
From the Finnish patent application number 974006 (Nokia Corp.) and the corresponding U.S. patent application Ser. No. 09/174,989 there is known a method and a system for processing an acoustic virtual environment. There the surfaces of the environment to be modeled are represented by filters having a certain frequency response. In order to transmit the modeled environment in digital transmission form it is sufficient to present in some way the transfer functions of all essential surfaces belonging to the environment. However, even this does not take into account the effects which the arrival direction or the pitch of the sound has on the direction of the sound.